2.3.3.2 General Escape Syntax

In addition to the specific escape sequences for special important
control characters, Emacs provides several types of escape syntax that
you can use to specify non-ASCII text characters.

You can specify characters by their Unicode names, if any.
?\N{NAME} represents the Unicode character named
NAME. Thus, ‘?\N{LATIN SMALL LETTER A WITH GRAVE}’ is
equivalent to ?à and denotes the Unicode character U+00E0. To
simplify entering multi-line strings, you can replace spaces in the
names by non-empty sequences of whitespace (e.g., newlines).

You can specify characters by their Unicode values.
?\N{U+X} represents a character with Unicode code point
X, where X is a hexadecimal number. Also,
?\uxxxx and ?\Uxxxxxxxx represent code
points xxxx and xxxxxxxx, respectively, where each x
is a single hexadecimal digit. For example, ?\N{U+E0},
?\u00e0 and ?\U000000E0 are all equivalent to ?à
and to ‘?\N{LATIN SMALL LETTER A WITH GRAVE}’. The Unicode
Standard defines code points only up to ‘U+10ffff’, so if
you specify a code point higher than that, Emacs signals an error.

You can specify characters by their hexadecimal character
codes. A hexadecimal escape sequence consists of a backslash,
‘x’, and the hexadecimal character code. Thus, ‘?\x41’ is
the character A, ‘?\x1’ is the character C-a, and
?\xe0 is the character à (a with grave accent).
You can use any number of hex digits, so you can represent any
character code in this way.

You can specify characters by their character code in
octal. An octal escape sequence consists of a backslash followed by
up to three octal digits; thus, ‘?\101’ for the character
A, ‘?\001’ for the character C-a, and ?\002
for the character C-b. Only characters up to octal code 777 can
be specified this way.